Kaolin is a naturally occurring, relatively fine, white clay which may be generally described as a hydrated aluminum silicate. Kaolin clay, after purification and beneficiation, is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
However, kaolin clay naturally contains discoloring impurities, such as oxides of titanium and iron. These oxides are largely responsible for the poor whiteness, brightness and color of the clay and are often the cause for rejecting a clay for commercial use. The use of kaolin as a coating pigment in the paper industry requires high brightness, proper color and gloss, which requires the removal of these discoloring impurities.
In general, wet beneficiation or purification of the kaolin to remove titanium and iron impurities involves high intensity magnetic separation, froth flotation, selective flocculation and/or leaching. The kaolin is initially dispersed in water, degritted (defined as removal of particles coarser than 44 microns) and then beneficiated in slurry form.
High intensity magnetic separation involves the use of a magnetic field to remove the impurities with magnetic susceptibility, such as anatase, rutile, hematite, mica and pyrite. However, this method is not very effective for submicron particles and thus the capability of this method to produce high brightness kaolin products is limited.
Froth flotation is an efficient method of removing titaniferous discoloring impurities from kaolin clays. In froth flotation, the impurity is rendered selectively hydrophobic by the addition of a collector (e.g., fatty acid, tall oil, hydroxamate, mixtures thereof, etc.). In some froth flotation processes, such as when tall oil is the collector, additional conditioning of the impurity with monovalent, divalent or trivalent cations may be necessary. The hydrophobic particles attach to the air bubbles and are separated from the hydrophilic kaolin in a froth flotation cell or column. Details of the flotation process to remove discoloring impurities from kaolin are described in U.S. Pat. Nos. 3,450,257; 3,979,282; 4,472,271; 4,492,628; 4,629,556; 5,522,986; 5,685,899 and 5,810,998.
A variation of froth flotation involves the use of carrier particles to improve fine particle flotation, as described in U.S. Pat. No. 2,990,958. However, due to the very fine particle size of kaolin clays, especially the fine-grained Tertiary kaolins, flotation is complicated and can be inefficient and/or costly.
The use of alkyl, aryl or alkylaryl hydroxamates in the flotation of minerals which chelate with hydroxamates is known in the industry (Nagaraj, Reagents in Minerals Technology, Chapter 9, pages 289-295, Marcel Dekker Inc., 1988). Hydroxamates are powerful collectors in flotation due to their ability to selectively chelate with minerals which contain titanium, yttrium, lanthanum, cerium, niobium, tantalum, calcium, tin, iron, manganese or copper. Mixtures of minerals containing copper and iron have been successfully beneficiated by flotation using hydroxamates as the collector (U.S. Pat. No. 3,438,494).
Flocculation involves the aggregation of fine particles which are suspended (i.e., dispersed) in liquid by a bonding agent (i.e., flocculent) that attaches to the particles. In general, the flocculant is initially adsorbed on the particles and bonds with adjoining particles. The bonded particles then form larger aggregates or flocs which settle or sediment out of the suspending liquid. Flocculants can be natural products such as starch, guar gum and alginates or synthetic polymers such as polyacrylamides, polyacrylates and polyethylene oxides. To achieve selectivity, the flocculant should only adsorb on certain types of mineral particles.
Selective flocculation is an effective process for recovering fine to ultrafine minerals that respond poorly to conventional beneficiation processes such as flotation and magnetic separation. The successful use of selective flocculation on mixtures of fine mineral particles such as kaolin clays, iron-bearing minerals, phosphates, potash, copper ores and coal is known in the industry.
Selective flocculation has been successful in beneficiating fine-grained kaolins. This process involves the activation of the impurity with polyvalent cations (U.S. Pat. Nos. 3,371,988; 3,701,417; 3,837,482 and 3,862,027), conditioning with ammonium salt (U.S. Pat. No. 4,604,369) or fatty acid and polyvalent cations (U.S. Pat. No. 5,535,890) and then selectively flocculating the impurities with anionic polymers. The drawback is the relatively low recoveries obtained in this process.
Selective flocculation is also being used to beneficiate other mixtures of mineral particles. Iron-bearing ores, specifically taconite, are commercially processed using selective flocculation. The iron ore is ground and then dispersed with caustic and sodium silicate. The dispersed ground ore is then selectively flocculated with a corn starch flocculant to separate hematite, an iron oxide mineral.
Phosphate minerals are beneficiated using selective flocculation to separate them from the associated clays, as described in U.S. Pat. No. 2,660,303. Potash is also beneficiated by selective flocculation using a nonionic polyacrylamide flocculant and/or ethoxylated alkylamic alkylguanidine complex. Another selective flocculation process to beneficiate alkaline carbonate minerals, phosphate minerals, zeolites and bauxites is described by U.S. Pat. No. 5,535,890. In this process, fatty acids and polyvalent cations are used to recondition the mineral suspension.
A recent development in selective flocculation is the use of hydroxamates for the beneficiation of mixtures of minerals which contain minerals which chelate with hydroxamates (U.S. patent application Ser. No. 09/045,842, filed Mar. 20, 1998 U.S. Pat. No. 6,041,939). The increased selectivity of hydroxamates for certain mineral phases in kaolin clays and other mixtures of mineral particles improves the process over selective flocculation processes utilizing other activators.
Another process involves leaching of the kaolin clay with iron-reducing reagents such as zinc or sodium hydrosulfite. This leaching method is limited to removing iron contaminants only. Other known leaching reagents and/or processes are not currently economical for removing titanium impurities.
Due to the limitations of these various beneficiation processes to separate certain minerals from mixtures of minerals, such as the discoloring impurities in kaolin clays, there is a need in the industry for a process that is more efficient and more cost effective.